Current United States Environmental Protection Agency (EPA) regulations for reference samplers designate the use of a "manual sampling" technique for the measurement of ambient solid particulate concentrations. "Manual sampling" essentially means that a filter is employed in a field unit to continuously sample airborne solid particulates therethrough over a defined period of time. The filter is then manually removed from the field unit and returned to a laboratory for exact measurement of particulate mass collected by the filter.
There are presently two major types of samplers which meet EPA approval as certified particulate reference samplers. These samplers are referred to as "high volume samplers" and "dichotomous samplers." High volume samplers, as the name implies, are systems which utilize a large throughput of air (e.g., 40 CFM) passing through a large filter (e.g., 8 inches.times.10 inches). The filter is laboratory preweighed and postweighed to determine the mass concentration of particulates within the ambient air over a typical sampling period (e.g., 24 hours). For postweighing, the filter is first reconditioned for, by way of example, a 24 hour period in a controlled temperature and humidity environment. High volume samplers are typically large and not easily transportable by one person. These samplers require the use of a large, high speed blower (pump) to maintain flow and generally require motor maintenance, and even rebuilding, at frequent intervals. If monitoring for more than one 24 hour period is desired, then multiple sampler units must be employed for unattended daily sampling without interruption, such as during a weekend or holiday. There is no standardized temperature history for particulates collected and each one of the multiple samplers is independently operated with no start/stop communication between them. Additionally, there is no provision for conditional sampling capability to allow automatic sampling during specific episodes of interest.
The other major technique employed today, referred to as "dichotomous sampling," operates in a similar fashion to high volume sampling, but utilizes a much lower flow (for example, 16.7 liters/minute), along with smaller filters (e.g., 37 mm in diameter). In a dichotomous sampler, two filters are used to collect particulates downstream from an internal flow divider. The flow stream aerodynamically separates the particulate into coarse (2.5 to 10 microns) and fine (2.5 micron and smaller) size fractions. Both the coarse and fine filters must be conditioned and weighed as noted above, with the sum of the mass on both filters being used along with the volume of air sampled to determine particulate concentration level.
Dichotomous sampling has many of the inherent disadvantages of current high volume sampling. For example, multiple dichotomous samplers must be employed at a monitoring site if extended sampling over a period of days is desired. Further, such samplers depend on flow restricters to maintain constant flow and require manual correction of pressure and temperature to estimate total flow. Large flow errors may occur as accumulated particulate increases the pressure drop across a filter. There is no sampling at standardized temperature and each sampler is independently operated with no stop/start communication between multiple samplers. Dichotomous samplers are not suited for conditional sampling and thereby lack the capability to automatically sample during specific episodes of interest (such as sampling only when the wind is coming from a certain direction or in response to other specific meteorological conditions). In addition, since a dichotomous sampler employs two small filters through which the flow is divided, a sensitive (and expensive) microbalance is required for a laboratory mass weighing accuracy which allows the sum of both filters to provide a particulate mass concentration result. A balance with one microgram sensitivity is typically required, which is a delicate and painstaking instrument to use on a routine basis.
In view of the drawbacks associated with present approaches, a new approach to manual particulate sampling which meets EPA regulations for reference samplers for collecting suspended (solid) particulate matter is needed. The ambient particulate monitor of the present invention provides such a new manual particulate sampler.